Implied symmetry Who understands it and uses it, or who is against it?
If by implied symmetry you mean placing centerlines on symmetric parts and only dimensioning one side, I use it. I've never had a problem with someone not understanding it.
I agree with Glenn Schroeder if that is you question, we do make sure have a (TYP) note on that sort of thing on the drawing for clarity sake.
Typ is not desired by ASME.
If a part has multiple features that are on both sides of the line of symmetry, only one side needs to have dimensions. If there are half of the features are on one side, only the amount of features on that one side need to be called out OR a construction line can be placed:
(SW doesn't have standard line for symmetry: Center-line with 2 lines on top or bottom) I have an ER for this: 1-8856474466
We dimension with implied symmetry and our people in our shop understand it.
Depends on what you mean by "symmetry" and "implied". If you mean that you have a part where the two sides are mirrors of each other, there is nothing in the standards that states a centerline down the middle implies anything. The general rule is that if something isn't implicit, then it's not stated at all. The solution in most cases is to apply GD&T to show exactly what you mean. All relationships between features can be covered without special notation or implied means using Positional and Profile tolerances.
As to Glenn's example, that is not implied, but rather another form of explicit dimensioning using linear dimensions.
Implied is the term used, but not the definition. There is nothing in the standards which defines "Implied Symmetry."
Matthew Lorono wrote: The general rule is that if something isn't implicit, then it's not stated at all. The solution in most cases is to apply GD&T to show exactly what you mean.
Matthew Lorono wrote:
The general rule is that if something isn't implicit, then it's not stated at all. The solution in most cases is to apply GD&T to show exactly what you mean.
If more than about 5% of the shop (or engineering for that matter) understood GD&T that would work. More people understand the CL as implying symmetry.
I do use them in the drawings and my manufacturer understands that well
Here is another post with similar discussions Implied Symmetry - Symmetrical Centerline in Solidworks
I sometimes add a note like the only below, but I usually don't bother. By the way, is there a particular reason you're asking?
As far as this goes as long as you have a working relationship(internal, or external) where this "standard" is accepted it is working.
If we receive a print with a centerline and only 1 side dimensioned we understand it to be symmetrical.
I believe someone earlier in this thread hit on another relative issue that has been cropping up lately(TYP) is no longer being accepted by many companies as a valid annotation.
As far as the ASME standards go I think I forgot about them the minute I left college due to the fact that I got "this is how we do it" at every job I have had since college.
Kudos for being ASME in school. I haven't heard of any one else being taught ASME or GD&T in school. Very amusing.
I am personally against it for several reasons.
1) If you are in QC, how do you measure to/from an imaginary centerline?
2) What is the tolerance on the location of this centerline?
3) A machinist or QC person is going to have to get out a calculator to figure out hole to hole distance, and doing that math has opportunity for human error and is a waste of their time.
4) If both holes are dimensioned off the centerline, you are getting double the listed tolerance. E.g. if the dimension is +/- .01, you're actually saying the holes can be +/- .02 apart, which may not really be what you mean. Maybe it is, but would someone else looking at your design be sure that's what you mean?
I use symmetry. I have had issues with vendors not understanding it. Some think it's either from CL of part, or CL of feature.
The drawing needs to be clear enough so not to question it.
ASME Y14.2 section 2.6: "Centerlines are used to represent axes or center planes of symmetrical parts or features . . ."
Both uses of a centerline appear equally valid. I see no difference between dimensioning between 2 holes and dimensioning between a hole and a center plane.
I find it useful to dimension across the center plane, rather than dimension to the center plane. That way, if you are dimensioning a feature of size, you can tolerance the dimension and add a location tolerance to it (see first image below, from ASME Y14.5, Fig. 7-65). If you are dimensioning a couple of symmetrically located holes, you can use a basic dimension (see second image below). Either way, you don't need a dimension to the center plane or to the perimeter.
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